The invention relates to a thermionic cathode comprising a cathode body consisting of a high-melting-point base material and a store of emitter material and an electron-emitting monolayer on the surface of the cathode body. The monolayer during operation of the cathode is replenished from the store of emitter material. The invention also relates to a method of manufacturing such a thermionic cathode. Such cathodes will hereinafter also be referred to as dispenser cathodes or monolayer cathodes.
Thermionic monolayer cathodes with thorium as an electron-emissive material or emissive material on tungsten as a high-melting-point base material or base matrix have long been known U.S. Pat. No. 1,244,216. Such cathodes have already been intensively investigated but due to their wide-spread commercial use because of their good vacuum behaviour, their very high emission and their favourable properties when used in UHF and microwave tubes, a further improvement in particular of the emission is necessary in view of the more stringent requirements.
Such thermionic monolayer cathodes generally consist of a base matrix of a high-melting-point-metal in which emitter material is incorporated elementarily or in the form of a compound. At the operating temperature the emitter material diffuses in the form of atoms to the surface of the cathode, for example, by grain boundary diffusion, volume diffusion or through pores, and forms or replenishes a surface monolayer. The mono-atomic layer of emitter atoms on the surface is supported by desorption. In the case of thoriated tungsten cathodes, Th is liberated from ThO.sub.2 thermally, and preferably by reaction with W.sub.2 C, and diffuses along the grain boundaries to the tungsten surface.
With a suitable choice of the emitter material and the base material, the dipole field between the monolayer and the underlying atoms of the base material generates an additional reduction of the emitter work function for thermionic electrons so that monolayer cathodes have a higher electron emission than cathodes of pure emitter material. For example, the work function for pure Th is approximately 3.5 eV, while for a Th monolayer on tungsten it is only 2.8 eV.
However, perfect operation of the cathode is obtained only when the overall emissive surface is covered by the mono-layer, that is by a mono-atomic film. This condition becomes critical at higher temperatures, at which a sufficient coating and hence emission is no longer ensured due to strong desorption of the emitter atoms. In the case of Th-[W] (thiorated tungsten) cathodes, such an emission decay occurs at approximately 2200 K. where the emission finally falls to that of pure tungsten. The temperature at which the emission decay occurs, however, does depend on the grain size, especially for dispenser-type cathodes with a monolayer replenished via grain boundary diffusion. Since the emitter atoms spread across the surface via surface diffusion, in which the sources of the emitter atoms are the grain boundaries, smaller crystallites lead naturally to a better coating with respect to equal diffusion length.
There has been an unsolved problem for decades regarding emission and thorium diffusion length. From measurements of the thorium desorption rates .nu..sub.D of tungsten and measurements of the surface diffusion constant D.sub..delta. for thorium on polycrystalline tungsten the diffusion length can be given as .sqroot.D.sub.o .multidot.c.sub.o /.nu..sub.D where c.sub.o =1 represents the relative Th concentration at the edge of the source. This theoretically required diffusion length, however, is some orders of magnitude larger than that which can be calculated from the average grain sizes and the temperature of the emission decay. I. Langmuir gave a possible explanation of this phenomenon by means of the so-called "boundary effect" (Journal of The Franklin Institute 217 (1934) 543-569). According to this article, increased thorium desorption occurs at the edges of the individual tungsten crystallites, that is to say at the thorium emanating places, for example, dependent on strongly inhomogeneous fields. This means of course an increase migration resistance and a shortening of the actual diffusion length. So it must be an object of a cathode improvement to obviate the boundary effect by suitable structuring of the cathode.
Besides the boundary effect, however, there is a further limitation of cathode emission to be eliminated. The subtractive dipole field between the emitter-monolayer and the base material depends considerably on the crystallite orientation of the base. In the usual polycrystalline non-textured cathodes, for example, in all conventional powder metallurgically manufactured monolayer cathodes, this leads to location dependent, strongly varying electron emission in which the lowest work function is achieved only in a few fortuitous, favourably oriented crystallites. So-called "Patchy emitters" are obtained.
From DE-OS No. 1439890, corresponding to U.S. Pat. No. 3,284,657, a method is known to coat conventional monolayer cathodes with a polycrystalline preferentially orientated layer, for example, of the base material, in which that preferential orientation of the coating layer is provided which causes the strongest reduction of the work function. In this manner, homogeneously emitting cathodes with increased emission current density are obtained to a good approximation, since all faces contribute to a similar extent to the emission. In the case of Th-[W] thoriated tungsten cathodes, for example, the &lt;111&gt; is the most favourable W orientation. However, the high electron emission of such preferentially oriented cathodes does not remain stable in time, the texture being partly destroyed even during activation.